Some motor vehicles have a function of temporarily automatically stopping an engine, for example, when waiting for a traffic light and automatically restarting the engine when starting, that is, a so-called no idling function. In such motor vehicles, a large current flows through a starter motor at the time of restarting the engine. Thus, a battery voltage largely drops, and onboard equipment may not normally operate. Thus, in order to compensate the drop in the battery voltage, a voltage conversion apparatus such as a boost DC-DC converter is provided.
The boost voltage conversion apparatus is generally provided with a booster circuit which includes a switching element, an inductor, and a diode. The boost voltage conversion apparatus switches an input voltage at a high speed by the switching, element to output the boosted voltage. In the booster circuit, runaway of a CPU which controls on and off operations of the switching element may cause an abnormality in a boost operation and overvoltage may thereby be output. Thus, the onboard equipment which receives voltage supply from the booster circuit may be broken or destroyed by an overvoltage.
JP 2014-13565 A, JP 2012-255949 A, and JP 2010-29009 A describe techniques for preventing overvoltage input. In JP 2014-13865 A, in order to prevent an electric shock and breakage of an element in replacing an LED lamp, there is provided overvoltage prevention means for reducing an open voltage between a pair of terminals to a predetermined voltage or less when an LED array is removed from the terminals.
JP 2012-253949 A describes a DC-DC converter provided with a first switch which is connected to a coil in series, a second switch which is connected to the coil in parallel, a first controller which controls on and off operations of the first switch based on an output voltage, and a second controller which interrupts the control of the first controller and controls on and off operations of the second switch when the output voltage rises to a predetermined value. The second controller includes a first comparator having a first hysteresis characteristic for the output voltage of the DC-DC converter and a second comparator having a second hysteresis characteristic for the output voltage of the DC-DC converter. Each of the comparators has upper and lower limit thresholds and outputs an H level signal or an L level signal based on a result of comparison between the output voltage of the DC-DC converter and the thresholds.
JP 2010-29009 A describes a power supply circuit provided with an oscillation circuit, a charge pump circuit, a reference voltage generation circuit, and an overvoltage protection circuit. The overvoltage protection circuit generates a proportional voltage which is proportional to an output voltage. When the proportional voltage becomes a reference voltage or larger, the overvoltage protection circuit determines that the output voltage has become an overvoltage and stops a boost operation of the charge pump circuit. The overvoltage protection circuit is provided with a comparator, a resistor, a capacitor, and an AND circuit. When the comparator performs voltage comparison between the reference voltage and the proportional voltage, a predetermined hysteresis is provided by the resistor, the capacitor, and the AND circuit.
When a hysteresis for the output voltage is provided as performed in JP 2012-253949 A and JP 2010-29009 A, it is possible to keep an average value of the output voltage within a rated value range. However, in JP 2012-253949 A and JP 2010-29009 A, a circuit configuration becomes complicated due to the comparator used for achieving the hysteresis.